[SI-LIST] : 20-H Rule and Self-Resonant Frequency of Power Planes

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From: jrbarnes@lexmark.com
Date: Thu Mar 30 2000 - 05:06:00 PST


SI-LIST'ers,
Page 54 of Mark I. Montrose's book "Printed Circuit Board Design Techniques for
EMC Compliance" (IEEE Press, 1996) has the following statement:
" Many multilayer PCBs generally have a self-resonant frequency between 200 and
400MHz. Use of the 20-H rule (see section 2.2) may increase the self-resonant
frequency by a factor of 2 to 3."

No where else have I found a mention of this effect. Following the 20-H rule
on a board with 0.010" spacing between layers would amount to making the power
plane 0.4" smaller in length and width than the ground plane. I don't see how
such a small change in dimensions could cause such a huge reduction in
inductance*capacitance that the second sentence suggests. Or could there be
some kind of impedance matching because of the difference in the fringe fields
at the edge of the power planes?

Have any of you observed this effect, in measuring actual boards or in
modelling? Thanks!

                                                   John Barnes Advisory
Engineer
                                                   Lexmark International

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